Paints and coatings are used to protect a surface from corrosion, oxidation or other types of deterioration and to provide decorative effects. Organic solvent-based paint or coating is a uniformly dispersed mixture ranging in viscosity from a thin liquid to a semi-solid paste and includes a film-forming polymeric binder, an organic solvent, pigment and other additives. The binder and the solvent collectively make what is known as the "vehicle."
Latex or emulsion paint is a water-based paint or coating of a dispersion of dry powders of pigment and dispersed colorant, plus various paint additives such as fillers and extenders, and a resin dispersion. Typically the dry powder dispersion is made by milling the dry ingredients into water. The resin dispersion is either a latex formed by emulsion polymerization or a resin in emulsion form. The two dispersions are blended to form an emulsion, or latex, paint. Thus, the binder for latex paints and coatings is in an aqueous-dispersed form, whereas in a solvent paint or coating, it is in a soluble form. Latex paints and coatings are popular consumer paints, as they are easy to apply, are easy to clean up, are nonflammable, generally lack a disagreeable odor and can be used on both interior and exterior surfaces.
Pigments impart color to both solvent and latex paints and coatings. They also contribute to the opacity, durability and hardness of paint coatings. They are added to paint in the form of dry pigment powders and pigmented tinting concentrate during paint manufacture at the paint plant. In addition, especially for consumer latex paints, they may be added in the form of pigmented paint tinting concentrates at the point of sale, such as at retail paint stores. Essentially, the consumer may choose a custom-made color of the paint by having the retailer add a tinting concentrate to a white or tintable base.
Pigments are ordinarily organic or inorganic dry powders which incorporate a mixture of primary particles, aggregates and agglomerates. Aggregates are primary pigment particles joined face-to-face, and agglomerates are primary pigment particles joined at an edge or a comer. For both industrial and consumer paints, whether solvent or latex, pigment must be dispersed homogeneously throughout the paint when added. To properly be dispersed, pigments must be wetted, deaggregated and deagglomerated in the paint vehicle. Dry pigments are insoluble in organic solvents and water, therefore necessitating wetting, deaggregation and deagglomeration before dispersion forces can take full effect and enable the production of a stable, colloidal pigmentary dispersion in the paint vehicle. The wetting or deaeration process is physical in nature, as it requires that the vehicle or solvent displace the air on the surface of the pigment particles. The ideal dispersion consists of a homogenous suspension of primary particles, after reducing any aggregates and agglomerates to primary particles.
The wetting process is accomplished through the use of wetting agents, which are a type of surface-active agent or "surfactant." As discussed, wetting involves incorporation of the pigment into the vehicle replacing pigment-air interfaces with pigment-vehicle interfaces. Surfactants tend to bring pigment into solution as a result of surface activity at the pigment-solution interface. In general, surfactant molecules are composed of two segregated portions, one of which has sufficient affinity for the solvent (organic or aqueous) to bring the entire molecule into solution. The other portion is rejected by the solvent, because it has less affinity for the solvent than the solvent molecules have for each other. If the forces rejecting this group are sufficiently strong, the solute molecule will tend to concentrate at an interface, so that at least part of the area of the rejected group is not in contact with the solvent molecules. While some organic solvents may be good pigment wetting agents themselves, surfactants are typically added to solvent paints to ensure thorough pigment dispersion throughout the paint vehicle. Water-based systems are poor wetting agents of dry pigments, and thus, latex paints necessarily require the addition of surfactants for pigment dispersion.
Surfactants also stabilize the pigment dispersion from reaggregating and reagglomerating. In strongly-polar organic solvents or water, electrostatic stabilization is imparted by charged species adsorbing onto the pigment surfaces. This causes a cloud of oppositely-charged counter-ions to form around the particle, thus making an electric double layer that will repel similarly-charged surfaces. In nonpolar systems, a steric-stability is imparted by the adsorbed dispersant that prevents close approach of the surfaces.
Surfactants conventionally used with tinting concentrates are primarily nonionic and amphoteric surfactants in conjunction with anionic surfactant. There are disadvantages with the known surfactants. Most notably, the surfactants known for use as pigment dispersants for both solvent, and latex paints or tinting concentrates therefor, can harm the rheological profiles of paints, tending to impart to a greater or lesser degree undesirable properties to the ultimately dried paint film. Surfactants can retard the curing or drying process of the paint and can increase the water sensitivity and reduce the scrub resistance of the dried film. Additionally, they can interfere with and hinder paint and pigment tinting concentrate thickeners and undesirably alter paint viscosity. Further, the primary nonionic surfactants used in conventional pigment dispersant formulations are alkyl phenol ethoxylates, which may be hazardous to human health.